Can I cover a gasline connection with a permanent ceiling like drywall or wood in a basement ceiling or (like electrical junction boxes) need to be exposed.

You cannot conceal a union. Gas approved fittings may be concealed, in fact, your house probably has fittings in the walls or ceiling from the meter to the heater, particularly if in an attic. Best bet is to go back to where a union is not concealed, open basement or crawl space. I believe NFPA 54 covers the installation of gas lines in residential units.